From an AFT press release:

CHICAGO—Teachers and staff in the one of city’s largest charter school networks overwhelmingly have chosen the Chicago Alliance of Charter School Teachers and Staff (Chicago ACTS), an affiliate of the 1.5 million-member American Federation of Teachers and the Illinois Federation of Teachers, as their bargaining agent.

The decision involves more than 400 teachers and staff in 13 schools operated by the United Neighborhood Organization. In March, UNO and the AFT reached a neutrality agreement under which UNO agreed not to take a position on whether its teachers and staff organized. Some 87 percent of the 415 workers who voted approved Chicago ACTS as their bargaining agent.

The decision by UNO employees to join Chicago ACTS means that more than 20 percent of Chicago’s charter school teachers and staff are now unionized—the highest union density where charter school employees do not automatically have a union.